Evaluation of Ground Rhyolite and Metakaolin in View of Preventing Delayed Ettringite Formation in the Long Term

In this study, rhyolite and metakaolin were evaluated in view of preventing delayed ettringite formation (DEF) in the long term. In the experimental study performed, mortar mixtures were produced by substituting cement with rhyolite and metakaolin at ratios of 5%, 10%, 15% and 20% by weight of cement. To make comparisons, reference mixtures without rhyolite and metakaolin were also produced. After steam curing application, Duggan cycle was applied to the mortar specimens to accelerate the delayed ettringite formation. SEM (Scanning Electron Microscope) and EDS (Energy Dispersive Spectrometry) analyses were carried out to determine the microstructure of the products formed after the cycle application and to characterize their composition. In SEM analysis, examinations were made on the prepared specimens in BSE (Backscattered Electron) mode. Needle-like, ball and massive ettringite structures were noted in the SEM-EDS examinations. Regardless of the rhyolite and metakaolin substitution ratio, DEF-induced expansions were almost at the same level and remained in the order of 0.06% until the end of 4 months. However, DEF-induced expansions were at the level of 0.12% at the end of 5 months, again independent of the rhyolite and metakaolin substitution, while this value reached the order of 0.14% at the end of 1 year. In the light of the measurements and analyses performed, the preventive effect of rhyolite on delayed ettringite formation was found remarkable. Limited to the findings obtained from this study, it can be concluded that rhyolite is at least as effective as metakaolin in terms of preventing delate ettringite formation. The most effective usage ratio determined separately for ground rhyolite and metakaolin in preventing DEF-induced expansion is found 10% by weight of cement.